1. Field of the Invention
This invention relates to a gasket material and, more particularly, to a non-asbestos formulation for a high temperature gasket.
2. Description of the Prior Art
Numerous types of gasketing material and gaskets are available as described, for example, in Perry, Chemical Engineer's Handbook, 3rd edition, page 342 of FIG. 88 thereof. These very commonly employ asbestos fibers combined in many instances with metallic structures.
It is well-known that, although asbestos fibers are resistant to high temperatures, they are affected by high temperatures with loss of flexibility because of loss of water and any volatile materials that may have been employed with them to facilitate fabrication. In general, all metallic gaskets, i.e., free from asbestos, are required for temperatures exceeding about 600.degree. C. However, metallic gaskets are generally of low resilience and, after formation, are ill-adapted to temperature cycling to elevated temperatures.
Fibrous materials have been incorporated to serve as reinforcing agents in amounts of from 5 to about 60% by weight of dry materials including chyrsotile or amphibole asbestos, alumina-silicate and other vitreous fibers (available under such tradenames as Fiberfrax, Kaowool, chopped E-glass), refractory filaments, e.g. chrystalline alumina whiskers and staple ceramic fibers and, in some cases, fine metallic filaments. It is desirable to avoid the use of asbestos because of the possible health hazard of this material. Metallic filaments are less desirable because of their very different coefficients of thermal expansion.
Binders have been used primarily to facilitate fabrication and are typically of two types. Organic binders are preferably elastomeric materials, usually incorporated as latices and deposited on the fibrous materials. Latices of natural and synthetic rubbers generally are useful and typical organic binders include polychloroprene, acrylonitrile rubbers, chloro-sulfonated polyethylene, polybutenes, phenol-formaldehyde thermosets, and the like.
Inorganic binders typically used include montmorillonite, e.g., bentonite, hectorite, or saponite and kaolinite (ball clay). Expanded vermiculite and synthetic mica microflakes have also been employed.
However, notwithstanding the recognized need for an asbestos-free formulation and the plethora of possible substitute materials, asbestos-free formulations have been found to have one or more defects such as low temperature tolerances due to the burn-out of organic materials, lack of resiliency, low resistance to absorption of fluid such as oil, low tensile strengths and insufficient cohesive strength to permit fabrication.